Radio frequency (RF) front-end circuits, such as transceivers, power amplifiers, and passives are increasingly used for wireless communication. The front-end passives include RF filters. RF front-end filters consisting of bulk acoustic wave (BAW) resonators have been proven to have a number of advantages regarding quality factor, power handling, ESD robustness and size over other technologies, such as surface acoustic wave (SAW) devices and ceramic filters. Temperature stable oscillator incorporating BAW resonator has also been demonstrated to be well suited for high-speed serial data applications, such as standard SATA hard disk drives, developing standard USB3 PC peripherals, and fiber optic transceivers.
Typically, a BAW resonator includes an acoustic reflector on which a piezoelectric film is sandwiched between two metal electrodes. FIG. 10 shows a conventional BAW resonator. The BAW resonator has a substrate 11, an acoustic reflector layer 12 formed on the substrate 11, a bottom electrode layer 13 formed on the acoustic reflector layer 12, a piezoelectric layer 14 formed on the bottom electrode layer 13, and a top electrode layer 15 formed on the piezoelectric layer 14. In practice, additional layers to the metal electrodes may be added to enhance resonator's functionality such as physical strength, passivation, temperature compensation and the like. When applying an alternating voltage at the resonant frequency between the two electrodes, a thickness longitudinal acoustic wave is formed in the piezoelectric layer/film and propagated along to the other layers in the BAW resonator. The function of the acoustic reflector is to create a very large acoustic impedance difference at the interface of the bottom electrode and the acoustic reflector, therefore a major portion of the acoustic wave energy is trapped in the resonator body containing the piezoelectric film and electrode layers. In one configuration, the acoustic reflector is formed of an air cavity. In another configuration, the acoustic reflector includes a plurality of alternating low and high acoustic impedance layers to isolate the resonator body from the bottom substrate to achieve the acoustic energy trapping in the resonator body. The latter type of BAW resonator is also referred to as solidly mounted resonator (SMR).
In operation, an alternating voltage is applied on the two electrodes of a BAW resonator and the electrical impedance of the BAW resonator is recorded with a sweep of frequency of the applied alternating voltage. The minimum and maximum of the curve of impedance magnitude correspond to series resonant frequency (fs) and parallel resonant frequency (fp) of the BAW resonator, respectively. The effective electromechanical coupling coefficient (Kt2eff) is calculated from the separation of the series resonant frequency and parallel resonant frequency. A larger separation of the series and parallel resonances indicates a greater Kt2eff, which is crucial to produce RF BAW filters with wide bandwidth. The width of the filter pass-band required for certain products define a lower limit for Kt2eff. A typical non-compensated BAW resonator has a Kt2eff value about 6% to 7%. Because a certain portion of acoustic wave energy is stored in the first several layers of the acoustic reflector close to the bottom electrode, the Kt2eff of an SMR is lower than that in an air-backed BAW resonator. Usually, a high value of Kt2eff is desired for filter applications, since higher Kt2eff improves insertion loss, and designers can trade off Kt2eff for the Q factor. In many cases, a small sacrifice in Kt2eff, gives rise to a large boost in the Q factor, thereby leading to steeper skirts and better immunity to frequency variations due to process—thus leading to better manufacturing yield.
Resonant frequency of a BAW resonator is determined by the thicknesses and acoustic velocities of all layers in the propagation path of the longitudinal acoustic wave. The resonant frequency is mainly impacted by the thickness and acoustic velocity of the piezoelectric layer. The thicknesses and acoustic velocities of both electrodes relatively strongly influence the resonant frequency. However, the acoustic reflector of air has a negligible effect on the resonant frequency because it reflects almost all the acoustic energy back to the piezoelectric film. In the case the acoustic reflector having a plurality of alternating low and high acoustic impedance layers, only the topmost layer of the reflector containing a small fraction of the acoustic energy contribute to the resonant frequency to some extent.
Both the thicknesses and acoustic velocities of the piezoelectric, metal or dielectric layers in the BAW resonator structure change as temperature varies, so does the resonant frequency of the BAW resonator. Although the thickness expansion or contraction of the layers with temperature change plays a role in the resonant frequency variation with the temperature change, the acoustic-wave traveling velocity change of the layers with the temperature change is the dominant factor of the BAW resonant frequency dependence on temperature. The acoustic velocity of propagation in most of the materials currently employed in BAW resonator exhibits a negative temperature coefficient, i.e., the acoustic velocity becomes smaller with the increase of temperature, because the materials become “softened” (e.g., the inter-atomic forces is weakened) at a higher temperature. A decrease in the inter-atomic force results in a decrease in the elastic constant of the material with a concomitant decrease in the acoustic velocity. For example, the temperature coefficient of the acoustic velocity of aluminum nitride (AlN) is about −25 ppm/° C., and the temperature coefficient of the acoustic velocity of molybdenum (Mo) is about −60 ppm/° C.
The temperature coefficient of frequency (TCF) of a BAW resonator constructed by a known plurality of layers is determined by the thicknesses of the layers and their relative position and role in the resonator acoustic stack. For example in a BAW resonator consisting of an AlN layer and two Mo electrodes, the TCF of the resonator is close to −25 ppm/° C. if the thicknesses of both Mo electrodes are much thinner than that of the AlN. In the case of which the thicknesses of Mo electrodes are comparable with that of AlN, and the temperature coefficient of Mo provides a greater contribution to the TCF of the BAW resonator. Consequently, the resonant frequency of such BAW resonators has a TCF in the range from around −30 ppm/° C. to −40 ppm/° C. The TCF of the resonator becomes more negative if the thickness ratio of Mo to AlN in the resonator structure is increased. RF filters with BAW resonators typically have a band pass frequency response, and the TCF of the BAW resonators causes a reduction of the manufacturing yield of the RF filters, because such temperature coefficient causes a reduction of the temperature range over which the device or component incorporating the BAW resonators meets its pass bandwidth specification. In the most demanding duplexer applications, a low TCF is very important as it allows achieving specification-compliance over a wider range of temperature. Highly stable oscillators incorporating the BAW resonators have a much more stringent demand on the TCF of the BAW resonators, an extremely low or approaching zero TCF is desirable, because most oscillators are used to provide reference or timing signals and an ultra small variation of these signals with temperature is required.
Therefore, it is desirable to maximize the Kt2eff of the BAW resonator while maintaining the good and stable temperature performance of the resonator. Hence, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.